Using a PCB as a heatsink

Hi - I'm going to be making a board that has a large number of high
power (1W) LEDs. I'd ideally like to use the PCB as a heatsink to get
rid of all the heat. How much surface area/watt do I need to allow?
Thanks!

Hi - I'm going to be making a board that has a large number of high
power (1W) LEDs. I'd ideally like to use the PCB as a heatsink to get
rid of all the heat. How much surface area/watt do I need to allow?
Thanks!

Hi - I'm going to be making a board that has a large number of high
power (1W) LEDs. I'd ideally like to use the PCB as a heatsink to get
rid of all the heat. How much surface area/watt do I need to allow?

Click to expand...

How many ? Use some simple logic and work out how hot your pcb is going to get.
Hint: 10W is a lot in an enclosed space.

How many ? Use some simple logic and work out how hot your pcb is going to get.
Hint: 10W is a lot in an enclosed space.

Click to expand...

Make small PCBs for each LED (1 sq cm ea or less), and mount those
above the driver board like a third of a cm or more with pins or stiff
wires, and the airgap will do a lot of cooling with a bit of flow.
Leave both sides of the tiny boards fully cladded on one or both sides
(of course negate shorts), and leave them bare ie no mask so they
radiate their heat better. Heavy gauge copper or SPC wire (solid)
would allow some heat conduction as well.

Hi - I'm going to be making a board that has a large number of high
power (1W) LEDs. I'd ideally like to use the PCB as a heatsink to get
rid of all the heat. How much surface area/watt do I need to allow?
Thanks!

Click to expand...

Hi Mike,

Power leds (1W and above) definitively need an aluminium subtrate (SMI type
PCB, ie PCB with the base epoxy replaced by aluminium), and this SMI needs
to be efficiently fitted on an appropriately calculated heatsink. Even
Alumium PCB alone is not sufficient, especially if you have several LEDs...
A good reference is Luxeon's "Thermal Design" application note available
here : http://www.luxeon.com/pdfs/AB05.pdf

If you can't design your own SMI board then the only reasonnable solution is
to by "level 2 LEDs", meaning LEDs already mounted on a small SMI, and to
fix these small SMIs on a good heatsink.

Make small PCBs for each LED (1 sq cm ea or less), and mount those
above the driver board like a third of a cm or more with pins or stiff
wires, and the airgap will do a lot of cooling with a bit of flow.
Leave both sides of the tiny boards fully cladded on one or both sides
(of course negate shorts), and leave them bare ie no mask so they
radiate their heat better. Heavy gauge copper or SPC wire (solid)
would allow some heat conduction as well.

Click to expand...

All good advice but you have to get the heat out of the enclosure eventually.

I unintentiomally left some paprework on top of my digital 'set top box' yesterday
and was astonished to see how warm it got. I had some picture breakup too which
alerted me to it.

The other important thing is to check how hot it gets *after several days* !

Make small PCBs for each LED (1 sq cm ea or less), and mount those
above the driver board like a third of a cm or more with pins or stiff
wires, and the airgap will do a lot of cooling with a bit of flow.
Leave both sides of the tiny boards fully cladded on one or both sides
(of course negate shorts), and leave them bare ie no mask so they
radiate their heat better. Heavy gauge copper or SPC wire (solid)
would allow some heat conduction as well.

Make small PCBs for each LED (1 sq cm ea or less), and mount those
above the driver board like a third of a cm or more with pins or stiff
wires, and the airgap will do a lot of cooling with a bit of flow.
Leave both sides of the tiny boards fully cladded on one or both sides
(of course negate shorts), and leave them bare ie no mask so they
radiate their heat better. Heavy gauge copper or SPC wire (solid)
would allow some heat conduction as well.

Click to expand...

Since this will obviously not work why go to all that trouble just to
destroy some leds?

You will need an mcpcb mounted to a heatsink.
Do the sums and work out the required theta J to A. Thence with the
knowledge of the led theta J to C the 'heatsinking' ability of the
mounting is revealed.
Look at the mcpcb data then calc the heatsink required.

Since this will obviously not work why go to all that trouble just to
destroy some leds?

You will need an mcpcb mounted to a heatsink.
Do the sums and work out the required theta J to A. Thence with the
knowledge of the led theta J to C the 'heatsinking' ability of the
mounting is revealed.
Look at the mcpcb data then calc the heatsink required.

Click to expand...

There is also conduction cooling through the large leads mentioned
into the main PCB as well as their radiating surfaces. This works.

One can also buy a bit of sheet copper, and solder a little U shaped
piece onto each PCB, or across all them, linking them together.

Since this will obviously not work why go to all that trouble just to
destroy some leds?

You will need an mcpcb mounted to a heatsink.
Do the sums and work out the required theta J to A. Thence with the
knowledge of the led theta J to C the 'heatsinking' ability of the
mounting is revealed.
Look at the mcpcb data then calc the heatsink required.

Click to expand...

That's funny. Looks to me like exactly what these guys did with one
of their one watt LEDs... you know Luxeon...

I'd have to try that. I'm a bit dubious. But radiation is a minor heat
dump mechanism at the sorts of temps that pcb's usually run at.

Click to expand...

Most of it at this level will/would get handled by thermal
conduction cooling via the leads into surrounding substrates.

That is why I think this would work so well. It has two soaking
elements instead of one, and if they were all on the main PCB the
whole mass could warm and actually degrade the thermal abatement
desires. PCB substrates are fiberglass and epoxy, and don't move
thermal energy very fast.

Heat radiating from a hot surface conducts into the air. Using the
single term radiates to air is fine. IR does the work either way,
from within the hot medium. How do you think heat conducts to air
down at that little boundary layer? Does it shake hands with the
surface? No. IR is the thermal engine of ALL matter. We wouldn't be
here otherwise.

And a solder mask adds a minute amount of thermal resistance, not
enough to worry about.

Click to expand...

It DOES matter when what WE are after is a maximized system for
removing heat.

Shall we discuss micro-air gaps under large IGBTs? Next thing,
you'll be telling me that doesn't matter either. There is an entire
industry that says you're wrong.

How about chemically blackening the copper ? Or a black solder resist?

Click to expand...

With emissivity the apparent color is not as important as the
surface quality.

We painted our black body calibration sources with a matte black
paint and got near .98 plus emissivity. Average Human skin is also at
..98

Both are due to surface texture. The bare aluminum ingot has a smooth
surface, and some of the IR generated inside when it gets heated get
reflected back inside. When it has the matte finish, it has many many
more angles to radiate from the surface at. They are little tiny
scratches that add up to more actual surface.

IR and a smooth, polished , flat surface do not get along. It
reflects back inside the medium, and any that does radiate only
radiates from the surface in a perpendicular ray. The matte surface
allows the heat in the medium to radiate away from its surface at
several angles of incidence. A denser IR "flux" emanates.

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